1. Field of the Invention
The present invention relates to a method for producing a ceramic circuit board More specifically, it relates to a method for producing a multilayer glass ceramic copper circuit board having an improved printing property and adhesion property, without adversely affecting the conditions of through-holes therein, by using a copper-base conductive paste composition
2. Description of the Related Art
Glass ceramic composite multilayer circuit boards having superior electrical characteristics can include a copper conductive layer, since the boards can be fired or baked at a relatively low temperature (e.g., 1000.degree. C. or less) to greatly improve the conductivity of the boards. However, known copper-base pastes used to form conductors have been developed mainly for use with alumina circuit boards and, therefore, have not been directly applied to multilayer glass-ceramic circuit boards. Nevertheless, the development of glass-ceramic circuit boards having good electric characteristics is progressing and, therefore, there is a need to develop conductive copper paste compositions suitable for use in glass-ceramic composite circuit boards in the form of a green sheet before firing, having good printing properties, excellent adhesion properties, and further, capable of entirely filling through holes in the boards.
Various conductive paste compositions are known and used in the art. For example, paste compositions containing a conductor component of a noble metal such as gold, silver, and palladium dispersed in organic vehicles, paste compositions containing a conductor component of a high-melting point metal such as tungsten and molybdenum dispersed in organic vehicles, and paste compositions containing a conductor component of copper dispersed in organic vehicles
Of the above-mentioned conductive paste compositions, the conventional copper paste compositions which can be fired at a relatively low temperature generally contain copper powder, glass powder, and an organic vehicle. Namely, generally 1% to 5% by weight, based on the weight of the copper powder (i.e., conductor component), of the glass powder is contained, to provide the required adhesion strength with the binder and the substrate, and the organic vehicle is contained so that the glass powder can be made into a paste by mixing therewith.
For example, JP-A-59-155988 (Kokai) discloses a method for producing a conductive paste composition for coating a thicker film or layer containing gold, silver, copper, platinum, palladium, nickel, and/or aluminum powder and an organic vehicle by mixing in the presence of a titanate coupling agent However, this paste is used only for alumina substrates and the titanate coupling agent is used to prevent the formation of metal flakes.
JP-A-58-74759 (Kokai) discloses a conductive copper paste composition containing copper powder, a thermosetting resin binder, an unsaturated fatty acid, and an organic titanium compound However, the organic titanium compound is used, together with the unsaturated fatty acid, to ensure a fine dispersion of the copper powder and to provide a stable and good conductivity.
U.S. Pat. No. 4,503,090 discloses a thick film resistor circuit formed from a paste containing copper, silver and aluminum, a binder, ethyl cellulose, and a solvent.
When the above known conductive paste compositions are used as a conductor component, however, the following disadvantages arise.
(1) The paste compositions containing a noble metal as a conductor component are expensive and, therefore, are not preferable in practice
(2) The paste compositions containing a metal having a high melting point cannot be fired at a relatively low temperature and, since the resistivity of the conductor component is high, the conductor has a poor quality.
(3) Although the development of paste compositions having copper as the conductor component is very desirable, the properties when printed on a green sheet of glass-ceramic composite substrate are poor and the adhesion strength after the integral firing is low. Furthermore, although it has been proposed that a small amount of glass having a low melting point be added, to improve the adhesion strength, this is not substantially effective for glass-ceramic composite substrates. Furthermore, when a small amount of glass powder having a low melting point is added, the copper powder has little or no flowability, and during the printing, the copper-base paste composition is not easily filled into through-holes in a glass-ceramic green sheet.